ABSTRACT

The historic Come-In-Time gold mine was developed in a ∼100*100 m area of the >7 km long orogenic Rise and Shine Shear Zone in Otago Schist. The oxidation of hydrothermal ankeritic and auriferous arsenopyrite and pyrite-bearing Otago Schist beneath a regional Miocene unconformity resulted in replacement of sulphides and ankerite by arsenic (As)-bearing ferric oxyhydroxide. The As-bearing ferric oxyhydroxide is mainly concentrated in and near the original mineralisation sites, especially shears, but mobilisation into post-mineralisation joints and fractures has redistributed As on a metre scale. Gold was liberated from sulphides during oxidation and aggregated by supergene oxidation processes into a smaller number of larger particles (mm scale) that are intimately intergrown with As-bearing ferric oxyhydroxide, especially in shears. Eluvial gold in proximal immature debris includes variably rounded supergene particles eroded from nearby, and some externally derived detrital flakes from previously overlying Miocene sediments. Some eluvial gold is trapped in fine sediments by lichen on outcrops. The hydrothermal As-Au relationship has therefore been broadly maintained through oxidation of rocks at the metre scale but Au concentrations have been locally enhanced in the surficial environment.

摘要

历史悠久的 Come-In-Time 金矿是在奥塔哥片岩 >7 公里长的造山隆起和 Shine 剪切带的约 100*100 米区域开发的。区域中新世不整合面下方的热液铁白云母和含金毒砂和含黄铁矿的奥塔哥片岩发生氧化，导致硫化物和铁白云石被含砷 (As) 的氢氧化正铁所取代。含砷的氢氧化正铁主要集中在原始矿化点及其附近，尤其是剪切带，但动员到矿化后的节理和裂缝中已经在米尺度上重新分布了砷。金在氧化过程中从硫化物中释放出来，并通过表生氧化过程聚集成较少数量的较大颗粒（毫米级），这些颗粒与含 As 的氢氧化氢氧化铁密切共生，尤其是在剪刀中。近端未成熟碎屑中的残积金包括从附近侵蚀的不同圆形表生颗粒，以及一些来自先前覆盖的中新世沉积物的外部碎屑片。一些残积金被露头上的地衣困在细小的沉积物中。因此，As-Au 热液关系通过米尺度的岩石氧化得到广泛维持，但 Au 浓度在地表环境中局部增强。